How do i cancil request for a certified tax form?

Who did you request it from?  The IRS?   Contact the IRS.  Not sure if they let you cancel----you will have to ask them.

To call the IRS:IRS:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

Listen to each menu before making the selection.

First choose your language. Press 1 for English.

Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.

Instead, press 2 for "personal income tax".

Then press 1 for "form, tax history, or payment".  

Then press 3 "for all other questions."

Then press 2 "for all other questions."  It should then transfer you to an agent.